1. Field of Technology
The present invention relates to a magnetic resonance image (MRI) superconductive magnet.
2. Prior Art
A MRI superconductive magnet uses liquid helium to cool the a superconductive magnet. The liquid helium has a boiling point of as low as about 4.2 K, and therefore evaporates even when a small amount of heat enters. A technology of means for reducing the amount of evaporation from liquid helium is well known as Gifford-McMahon (GM) refrigerator which is used to recondense the evaporated gas to liquid and restore said liquid.
As for a magnetic shield as described in the Patent Document 1, “Superconductive Magnet”, if electromagnetic force generated by a superconductive coil causes force to be applied to a displacer in which a magnetic substance is accommodated, a large load is applied to a motor for driving the displacer, and thus the refrigerator does not operate correctly. To avoid this problem, the magnetic shield is used to assure normal operation of the refrigerator by blocking the magnetic field of the superconductive magnet. The material of the magnetic shield used is iron.
If the magnetic shield is used for the MRI superconductive magnet, not only does the unit become large, but also its weight may increase because the magnetic shield is made of iron and must be thicker for use in a high magnetic field.
[Patent Document 1] Japanese Application Patent Laid-Open Publication No. 2004-235653